Abstract
We use unpolarized and polarized inelastic neutron scattering to study low-energy spin excitations in NaFeAs, which exhibits a tetragonal-to-orthorhombic lattice distortion at K followed by a collinear antiferromagnetic (AF) order below K. In the AF ordered state (), spin waves are entirely -axis polarized below 10 meV, exhibiting a gap of 4 meV at the AF zone center and disperse to 7 meV near the -axis AF zone boundary. On warming to the paramagnetic state with orthorhombic lattice distortion (), spin excitations become anisotropic within the FeAs plane. Upon further warming to the paramagnetic tetragonal state (), spin excitations become more isotropic. Since similar magnetic anisotropy is also observed in the paramagnetic tetragonal phase of superconducting BaFeNiAs, our results suggest that the spin excitation anisotropy in superconducting iron pnictides originates from similar anisotropy already present in their parent compounds.
- Received 22 May 2013
DOI:https://doi.org/10.1103/PhysRevB.88.134512
©2013 American Physical Society